Improved electrical connections to dielectric materials

Abstract: A preferred embodiment of this invention comprises an oxidizable layer (e.g. tantalum 48 ), an oxygen gettering layer (e.g. platinum/tantalum mixture 34 ) overlaying the oxidizable layer, a noble metal layer (e.g. platinum 36 ) overlaying the oxygen gettering layer, and a high-dielectric-constant material layer (e.g. barium strontium titanate 38 ) overlaying the noble metal layer. The novel structures presented provide electrical connection to high-dielectric-constant materials without the disadvantages of current structures. The oxygen gettering layer controls oxygen diffusion, minimizing the formation of a resistive layer either in the lower electrode or at the lower electrode/substrate interface. The oxygen gettering layer acts as a gettering site for oxygen, where the oxygen oxidizes the reactive metal portion of the layer, leaving the noble metal portion of the layer intact. While the oxides/suboxides (e.g. tantalum pentoxide 40 ) that are formed are resistive, they are dispersed within the noble metal matrix, leaving a conductive path from the top of the layer to the bottom. This invention provides a stable and electrically conductive electrode for high-dielectric-constant materials while using standard integrated circuit materials to facilitate and economize the manufacturing process.

Description: FIELD OF THE INVENTION This invention generally relates to improving electrical connections to materials with high-dielectric-constants, such as in the construction of capacitors. BACKGROUND OF THE INVENTION Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to high-dielectric-constant materials, as an example. The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area/unit area by increasing the topography, such as in trench and stack capacitors using SiO 2 or SiO 2 /Si 3 N 4 as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs. An alternative approach is to use a high permittivity dielectric material. To be useful in electronic devices, however, reliable electrical connections should generally be constructed which do not diminish the beneficial properties of these high-dielectric-constant materials. Heretofore in this field, single and multiple metal layers are generally used to form electrical contacts to high-dielectric-constant materials. For example, to provide an electrical connection to a high-dielectric-constant material which makes up a capacitor on the surface of a semiconductor substrate, the following techniques are among those now employed: (a) dielectric-platinum-substrate; (b) dielectric-platinum-tantalum-substrate; (c) dielectric-platinum-titanium-substrate. The layering sequence in these examples is from the top down to the substrate (e.g. silicon). A similar metalisation scheme may be used for the top of the dielectric film to complete the capacitor structure. EP-A 0 404 295 discloses a multi-layer capacitor with a dielectric-platinum-titanium-substrate structure in which the titanium layer is deliberately oxidised to titanium oxide in order to improve the adhesion of the platinum. The platinum layer is the operative electrode, which is connected to other electrical circuitry. International Electron Devices Meeting 1991, pages 823-826, describes a dielectric-platinum-tantalum-substrate structure in which the electrical connection to the bottom electrode is through the tantalum layer. The tantalum layer is said to prevent undesirable diffusion of the silicon upwards into the dielectric, but there is no suggestion that oxygen diffuses downwards into the tantalum layer. The invention provides a method of forming a microelectronic structure, comprising: forming an oxidisable layer; forming on the oxidisable layer an oxygen-gettering layer comprising a mixture of an oxygen-gettering reactive component and a non-reactive noble metal ; forming a noble metal layer on the oxygen-gettering layer; and forming a layer of a high-dielectric-constant material on the noble metal layer. The invention also provides a microelectronic structure, comprising: an oxidisable layer; an oxygen-gettering layer overlying the oxidisable layer comprising a mixture of at least one oxygen-gettering reactive component and at least one non-reactive noble metal component; a noble metal layer overlying the oxygen-gettering layer; and a layer of a high-dielectric-constant material overlying the noble metal layer. As used herein, the term "high-dielectric-constant" (hereafter abbreviated to HDC) means a dielectric constant greater than about 20. HDC materials are useful for the fabrication of many electrical devices, such as capacitors. However, HDC materials are generally not chemically stable when deposited directly on a semiconductor substrate, so one or more additional layers are required to provide the electrical connection between the HDC material and the substrate. The additional layer or layers should generally be chemically stable when in contact with the substrate and also when in contact with the HDC material. The said reactive component and the said oxidisable layer may be selected from the group consisting of: tantalum, tungsten, titanium, molybdenum, titanium nitride, zirconium nitride, titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide, tantalum carbide, titanium boride, boron carbide, silicon, germanium, carbon, GaAs, and combinations thereof. Current methods provide for using platinum as the noble layer to contact the high-dielectric-constant material, along with tantalum or titanium as the sticking layer to contact the substrate. However, HDC materials (e.g. ferroelectrics) are generally deposited at elevated temperatures (greater than about 500°C) in an O 2 atmosphere. It has been discovered that, in this environment, oxygen diffuses through the platinum and forms a resistive layer of Ta 2 O 5 or TiO 2 when it comes in contact with the Ta or Ti, causing high contact resistance. Further, the substrate (e.g. silicon) itself can become oxidized during the deposition of the HDC material. As used herein, the term oxidizable layer refers to the underlying sticking layer, or substrate, which becomes more insulating when oxidized. The disclosed structures generally provide electrical connection to HDC materials without the disadvantages of the current structures. One embodiment of this invention comprises an oxidizable layer, an oxygen gettering layer overlaying the oxidizable layer, a noble metal layer overlaying the oxygen gettering layer, and a high-dielectric-constant material layer overlaying the noble metal layer. A method of forming an embodiment of this invention comprises the steps of forming a oxygen gettering layer on an oxidizable layer, forming a noble metal layer on the oxygen gettering layer, and forming a high-dielectric-constant material layer on the noble metal layer. Examples of processes for depositing the lower electrode materials are sputtering, evaporation, and chemical vapor deposition. Examples of processes for depositing the high-dielectric-constant material are ion-beam sputtering, chemical vapor deposition, and pulsed laser deposition. These are apparently the first structures wherein an electrical connection to high-dielectric-constant materials comprises an oxygen gettering layer. The oxygen gettering layer controls oxygen diffusion, minimizing the formation of a resistive layer either in the oxidizable layer or at the oxidizable layer/oxygen gettering layer interface. The oxygen gettering layer acts as a gettering site for oxygen, wherein the oxygen oxidizes the reactive metal portion of the layer, leaving the noble metal portion of the layer intact. While the oxides or suboxides that are formed are resistive, they are dispersed within the noble metal matrix, leaving a conductive path from the top of the layer to the bottom. The oxygen gettering layer should generally contain enough reactive metal to successfully combine with most or all of the diffused oxygen, but not so much that there is not a conductive path remaining via the noble metal component of the layer. Generally, the required thickness and composition of the oxygen gettering layer depend on the specific deposition parameters (temperature, O 2 pressure, etc.) of the high-dielectric-constant material. If enough oxygen reaches the oxidizable layer (e.g. the sticking layer or the substrate), a resistive layer will be formed, significantly increasing the contact resistance. The noble metal layer between the high-dielectric-constant material layer and the oxygen gettering layer is desirable, as it both minimizes undesirable reduction of the high-dielectric-constant material layer and lowers the amount of oxygen which enters the oxygen gettering layer. This invention generally provides a stable electrode for HDC materials while using standard integrated circuit materials to facilitate and economize the manufacturing process. BRIEF DESCRIPTION OF THE DRAWINGS The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein: FIGS. 1-5 are cross-sectional views of a method for constructing a metal/high-dielectric-constant material/metal capacitor on a semiconductor substrate; FIGS. 6-9 are cross-sectional views of metal/high-dielectric-constant material/metal capacitors formed on the surface of a semiconductor substrate; FIG. 10 is a cross-sectional view of an intermediate structure, a lower electrode formed on the surface of a semiconductor substrate, before the deposition of a high-dielectric-constant material layer; FIG. 11 is a cross-sectional view of an oxygen gettering layer providing electrical connection between a high-dielectric-constant material layer and an oxidizable layer; FIG. 12 is a cross-sectional view of a metal/high-dielectric-constant material/metal capacitor formed on the surface of a semiconductor substrate; and FIG. 13 is a cross-sectional view of a lower electrode providing electrical connection between a high-dielectric-constant material and a semiconductor substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to Figures 1-5, there is shown a method of forming a preferred embodiment of this invention, a capacitor comprising a high-dielectric-constant material and an oxygen gettering layer. Figure 1 illustrates a tantalum sticking layer 32 deposited on the surface of a semiconductor body 30. Figure 2 illustrates a platinum/tantalum mixture layer 34 deposited on the tantalum sticking layer 32. The ratio of platinum to tantalum in the platinum/tantalum mixture layer 34 is, in this example, between 3:1 and 1:1.5. A method of forming this oxygen gettering layer 34 involves depositing the platinum and tantalum in the same process chamber using two sputter guns, so that the desired percentages of each metal are deposited, forming a mixture of the two metals. Other processes such as evaporation or chemical vapor deposition could also be used to deposit the two metals. Figure 3 illustrates a platinum noble layer 36 deposited on the oxygen gettering layer 34. The three layers 32, 34, and 36 constitute the lower electrode of the capacitor. Figure 4 illustrates a barium strontium titanate layer 38 deposited on the platinum layer 36, and also the tantalum oxide particles 40 that are formed in the oxygen gettering layer 34 as a consequence of the barium strontium titanate layer 38 being deposited in a high temperature oxygen atmosphere. The lower electrode, comprising layers 32, 34, and 36, remains electrically conductive after the barium strontium titanate deposition since the oxide particles 40 are spread throughout the oxygen gettering layer 34. The barium strontium titanate layer 38 constitutes the dielectric of the capacitor. Figure 5 illustrates a titanium nitride layer 42 deposited on the barium strontium titanate layer 38, forming the upper electrode of the capacitor. Although Figure 5 illustrates a planar geometry for all of the elements of the capacitor, this invention applies equally well to the construction of capacitor structures of more complicated geometries, including capacitors built into depressions in the semiconductor surface. In an alternate embodiment, with reference to Figure 6, such a capacitor structure may be formed without the tantalum layer 32 of Figure 5, and thus the oxygen gettering layer is preventing the oxidation of the top of the substrate. In another alternate embodiment, with reference to Figure 7, such a capacitor structure may be formed without the platinum layer 36 of Figure 5. In another alternate embodiment, with reference to Figure 8, such a capacitor structure may be formed without either the tantalum layer 32 or the platinum layer 36 of Figure 5. In another alternate embodiment, with reference to Figure 9, such a capacitor structure may be formed with a lower electrode comprising a graded layer 44 in which the amounts of each metal vary with respect to depth in the layer. The graded layer 44 is formed such that primarily tantalum is deposited near the semiconductor substrate, and then as the layer continues to be formed, the amount of platinum deposited is increased while the amount of tantalum deposited is decreased. Near the top of the layer, primarily platinum is deposited. In another alternate embodiment, with reference to Figure 10, such a capacitor may be formed with a lower electrode comprising a stratified region 46 between the tantalum layer 32 and the platinum layer 36. This stratified region 46 comprises relatively thin alternating layers of platinum and tantalum, which will intermix when heated, dispersing the tantalum into the platinum. The number and thickness of the layers in the stratified region 46 can be varied, depending on the amounts of tantalum and platinum required. Figure 10 depicts the lower electrode before application of the HDC material. In another alternate embodiment, with reference to Figure 11, the oxygen gettering layer 34 may be formed on an oxidizable layer 48. A platinum noble layer 36 is formed on the oxygen gettering layer 34, and a barium strontium titanate layer 38 is formed on the platinum layer 36, which causes the oxide particles 40 to form. In another alternate embodiment, with reference to Figure 12, the tantalum sticking layer 32 may be formed on a titanium nitride barrier layer 52, which is itself formed on a titanium silicide contact layer 50. The titanium nitride layer 52 and titanium silicide layer 50 are formed by depositing a thin layer of titanium on the silicon substrate 30, and then annealing in an NH 3 atmosphere. The titanium silicide layer 50 forms a low resistance contact to the underlying silicon substrate 30, while the titanium nitride layer 52 prevents the formation of insulating tantalum silicide between the silicon substrate 30 and the tantalum sticking layer 32. In yet another alternate embodiment, with reference to Figure 13, a tantalum plug 56 is formed through a silicon dioxide insulating layer 54, connecting the platinum/tantalum mixture layer 34 to the underlying silicon substrate 30. The sole Table, below, provides an overview of some embodiments and the drawings. Drawing Element Generic Term Preferred or Specific Examples Other Alternate Examples 30 Semiconductor Substrate Silicon Other single component semiconductors (e.g. germanium) Compound semiconductors (e.g GaAs, Si/Ge, Si/C) May be the oxidizable layer (e.g. if no sticking layer is used 32 Sticking layer (when used, is generally the oxidizable layer) Tantalum Other reactive metals (e.g. tungsten, titanium, molybdenum). Reactive metal compounds (e.g. nitrides: titanium nitride, zirconium nitride; silicides: titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide; carbides: tantalum carbide; borides: titanium boride) Conductive carbides and borides (e.g. boron carbide) 34 Oxygen gettering layer Platinum/ tantalum mixture wherein the ratio of platinum to tantalum is between 3:1 and 1:1.5 Noble metal/reactive metal mixtures wherein the ratio of noble metal to reactive metal is between 99:1 and 1:3 (e.g. other platinum/tantalum mixtures, platinum/tungsten mixtures, platinum/titanium mixtures) Noble metal/reactive metal compound mixtures wherein the ratio of noble metal to reactive metal compound is between 99:1 and 1:3 (e.g. platinum/titanium nitride mixtures) Other combinations of above mentioned materials (e.g. platinum/tantalum/tungsten mixture) Other combinations of materials selected from Drawing Element 32 above and Drawing Element 36 below (e.g. palladium/molybdenum mixtures) 36 Noble layer Platinum Other noble metals (e.g. palladium, rhodium, gold, indium, silver) 38 High-dielectric-constant material Barium strontium titanate Other transition metal titanates, tantalates, niobates, and zirconates (e.g. barium titanate, strontium titanate, lead zirconate titanate, lead zinc niobate) Other high dielectric constant oxides (e.g. tantalum pentoxide) 40 Oxide particles Tantalum pentoxide Other oxides/suboxides of reactive metals and reactive metal compounds (e.g. other tantalum oxides, tungsten oxides, titanium oxides) 42 Upper electrode Titanium nitride Other metal nitrides Ruthenium dioxide YBa 2 Cu 3 O 7-x Noble metals (e.g. platinum, palladium, rhodium, gold, iridium, silver) Other common semiconductor electrodes (e.g. silicides, aluminum) 44 Graded layer Platinum/tantalum mixture wherein the mixture is 100% tantalum near the substrate and transitions to 100% platinum near the HDC material Other noble metal/reactive metal mixtures wherein the mixture is 100% reactive metal near the substrate and transitions to 100% noble metal near the HDC material (e.g. platinum/tungsten mixture, platinum/titanium mixture) Noble metal/reactive metal compound mixtures wherein the mixture is 100% reactive metal compound near the substrate and transitions to 100% noble metal near the HDC material (e.g. platinum/titanium nitride mixture) Other combinations of above mentioned materials (e.g. platinum/ tantalum/tungsten mixture) Other combinations of materials selected from Drawing Element 32 above and Drawing Element 36 above (e.g. palladium/molybdenum mixtures) 46 Stratified region Alternating layers of platinum and tantalum wherein the thickness ratio of the platinum layers to the tantalum layers is between 3:1 and 1:1.5 Alternating layers of other noble and reactive metals wherein the thickness ratio of the noble metal layers to the reactive metal layers is between 99:1 and 1:3 (e.g. platinum/tantalum, platinum/tungsten, platinum/ titanium) Alternating layers of noble metal and reactive metal compound wherein the thickness ratio of the noble metal layers to the reactive metal compound layers is between 99:1 and 1:3 (e.g. platinum/titanium nitride) Other combinations of above mentioned materials (e.g. platinum/ tantalum/tungsten) Other combinations of materials selected from Drawing Element 32 above and Drawing Element 36 above (e.g. palladium/molybdenum) 48 Oxidizable layer Tantalum Other reactive metals (e.g. tungsten, titanium, molybdenum) Reactive metal compounds (e.g. nitrides: titanium nitride, zirconium nitride; silicides: titanium silicide, tantalum silidde, tungsten silicide, molybdenum silicide; carbides: tantalum carbide; borides: titanium boride) Conductive carbides and borides (e.g. boron carbide) Single component semiconductors (e.g. single crystalline and polycrystalline silicon, germanium) Compound semiconductors (e.g. GaAs, Si/Ge, Si/C) 50 Contact layer Titanium silicide Other conductive silicides 52 Barrier layer Titanium nitride Other conductive nitrides Other high-temperature conductive diffusion barriers 54 Insulator Silicon dioxide Other insulators (e.g. silicon nitride) 56 Conductive Plug Tantalum Other reactive metals (e.g. tungsten, titanium, molybdenum) Reactive metal compounds (e.g. nitrides: titanium nitride, zirconium nitride; silicides: titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide; carbides: tantalum carbide; borides: titanium boride) Conductive carbides and borides (e.g. boron carbide) Single component semiconductors (e.g. single crystalline and polycrystalline silicon, germanium) Compound semiconductors (e.g. GaAs, Si/Ge, Si/C) A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. With reference to the structures described, electrical connections to such structures can be ohmic, rectifying, capacitive, direct or indirect, via intervening circuits or otherwise. Implementation is contemplated in discrete components or fully integrated circuits in silicon, germanium, gallium arsenide, or other electronic materials families. In general the preferred or specific examples are preferred over the other alternate examples. Unless otherwise stated, all composition ratios or percentages are in relation to composition by weight. In some intermediate structures, and in the final product, the oxygen gettering layer will generally be at least partially oxidized. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense.

Claims: A method of forming a microelectronic structure, comprising: forming an oxidisable layer (30, 32); forming on the oxidisable layer (30, 32) an oxygen-gettering layer (34) comprising a mixture of at least one oxygen-gettering reactive component and at least one non-reactive noble metal; forming a noble metal layer (36) on the oxygen-gettering layer (34); and forming a layer of a high-dielectric-constant material (38) on the noble metal layer (36). A method according to claim 1, wherein the layer of a high-dielectric-constant material (38) is formed by a process that oxidises the reactive component of the oxygen-gettering layer (34). A method according to claim 1 or claim 2, comprising forming an electrically conductive layer (42) on the layer of high-dielectric-constant material. A method according to claim 3, comprising selecting the said electrically conductive layer (42) from the group consisting of titanium nitride, ruthenium dioxide, YBa 2 Cu 3 O 7-x , platinum, palladium, rhodium, gold, iridium, silver, and combinations thereof. A method according to any preceding claim, comprising forming said oxygen-gettering layer (34) such that the ratio of the noble metal to the reactive component is between about 99:1 and about 1:3. A method according to claim 5, comprising forming the oxygen-gettering layer (44) comprising a graded mixture of the noble metal and the reactive component, wherein the reactive component constitutes more than 99% of the oxygen-gettering layer near the oxidisable layer (30, 32), and the reactive component transitions to constitute less than 1% of the oxygen gettering layer near the noble metal layer (36). A method according to claim 5 or claim 6, comprising selecting said reactive component and said oxidisable layer (30, 32) from the group consisting of: tantalum, tungsten, titanium, molybdenum, titanium nitride, zirconium nitride, titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide, tantalum carbide, titanium boride, boron carbide, silicon, germanium, carbon, GaAs, and combinations thereof A method according to any preceding claim, comprising forming the said noble metal layer from the group consisting of platinum, palladium, rhodium, gold, iridium, silver, and combinations thereof A method according to any preceding claim, comprising selecting the said high-dielectric-constant material (38) from the group consisting of: barium strontium titanate, barium titanate, strontium titanate, lead zirconium titanate, lead zinc niobate, tantalum pentoxide, and combinations thereof A method according to any preceding claim for forming a microelectronic capacitor layer. A microelectronic structure, comprising: an oxidisable layer (30, 32); an oxygen-gettering layer (34, 44) overlying the oxidisable layer (30, 32) comprising a mixture of at least one oxygen-gettering reactive component and at least one non-reactive noble metal component; a noble metal layer (36) overlying the oxygen-gettering layer (34, 44); and a layer of a high-dielectric-constant material (38) overlying the noble metal layer (36). A microelectronic structure according to claim 11, wherein the reactive component is at least partially oxidised, and the said noble metal component provides electrical conductivity through the oxygen-gettering layer (34, 44). A microelectronic structure according to claim 11 or claim 12, comprising an electrically conductive layer (42), preferably selected from the group consisting of titanium nitride, ruthenium dioxide, YBa 2 Cu 3 O 7-x , platinum, palladium, rhodium, gold, iridium, silver, and combinations thereof, overlying the layer of high-dielectric-constant material. A microelectronic structure according to any of claims 11 to 13, wherein the ratio of the noble metal component to the reactive component in the said oxygen-gettering layer (34,44) is between about 99:1 and about 1:3 A microelectronic structure according to claim 14, wherein the oxygen-gettering layer (44) comprises a graded mixture of the noble metal component and the reactive component, wherein the reactive component constitutes more than 99% of the oxygen-gettering layer near the oxidisable layer (30, 32), and the reactive component transitions to constitute less than 1% of the oxygen-gettering layer near the noble metal layer (36). A microelectronic structure according to claim 14 or claim 15, wherein said reactive component and said oxidisable layer (30, 32) are selected from the group consisting of: tantalum, tungsten, titanium, molybdenum, titanium nitride, zirconium nitride, titanium silicide, tantalum silicide, tungsten silicide, molybdenum silicide, tantalum carbide, titanium boride, boron carbide, silicon, germanium, carbon, GaAs, and combinations thereof. A microelectronic structure according to any one of claims 11 to 16, wherein the said high-dielectric-constant material (38) is selected from the group consisting of: barium strontium titanate, barium titanate, strontium titanate, lead zirconium titanate, lead zinc niobate, tantalum pentoxide, and combinations thereof.

Inventors: GNADE BRUCE E, SUMMERFELT SCOTT R, GNADE,BRUCE E., SUMMERFELT, SCOTT R.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, H01L27